<!DOCTYPE html>
<html lang="en">
<head>
  <meta charset="UTF-8">
  <meta name="viewport" content="width=device-width, initial-scale=1.0">
  <meta http-equiv="X-UA-Compatible" content="ie=edge">
  <title>Document</title>
</head>
<body>
  <div class="assistor">
    <div class="parent">
      <div class="fixed-child">
        This is header
      </div>
      <div class="content">
        <p>This is a paragraph</p>
        <p>This is a paragraph</p>
        <p>This is a paragraph</p>
        <p>This is a paragraph</p>
        <p>This is a paragraph</p>
        <p>This is a paragraph</p>
        <p>This is a paragraph</p>
        <p>This is a paragraph</p>
        <p>This is a paragraph</p>
        <p>This is a paragraph</p>
        <p>This is a paragraph</p>
        <p>This is a paragraph</p>
      </div>
    </div>
  </div>
  <style>
    .assistor {
      position: relative;
      width: 500px;
      height: 100px;
      margin: 100px auto 0 auto;
      background: #ddd;
    }

    .parent {
      width: 500px;
      height: 100px;
      overflow: auto;
      background: #bbffaa;
    }

    .fixed-child {
      position: absolute;
      width: 100%;
      height: 25px;
      text-align: center;
      background: #f0f0f0;
    }

    .content {
      margin-top: 25px;
    }
  </style>
</body>
</html>